"address_space()" definition as a "Sparse" annotation in the linux kernel

I came across some macros with the definitions like this:

#ifdef __CHECKER__# define __user     __attribute__((noderef, address_space(1)))

And I know those are merely to be used with sparse(__CHECKER__) and GCC ignores them completely.But what I don't get is that what does address_space() actually mean? What are the possible values for it? Looks like the only documentation for it is a post from linus in the mailing list around 16 years ago which says:

When you do use parse, it is another matter entirely. For"sparse", that "__iomem" has lots of meaning:
# define __iomem __attribute__((noderef, address_space(2)))

ie "iomem" means two separate things: it means that sparse should complainif the pointer is ever dereferenced (it's a "noderef" pointer) directly,and it's in "address space 2" as opposed to the normal address space (0).

From similar Q&As like this and this I figured out that 3 means per-cpu pointers and 1 seems to be related to the pointers received from userspace.And also 0 for normal pointers.

But what does address_space(2) actually mean?
Are {0,1,2,3} the only possible values?

Thanks.